A PRAM device may store data by using a resistance difference between an amorphous state and a crystalline state according to a phase change of a variable resistance material layer. A phase of the variable resistance material layer may change from a crystalline state into an amorphous state at a temperature equal to or higher than the melting temperature thereof, and may change from the amorphous state into the crystalline state at a temperature equal to or higher than the crystallization temperature thereof. The melting temperature may be higher than the crystallization temperature.
When current flows in a variable resistance material layer, Joule's heat may be generated in the variable resistance material layer. The Joule's heat generated in the variable resistance material layer may change the phase of the variable resistance material layer from an amorphous state into a crystalline state or from the crystalline state into the amorphous state. That is, when current flows in a variable resistance material layer, the writing speed of a PRAM device may be determined by the phase change speed (e.g., crystallization rate) of the variable resistance material layer. When current does not flow in the variable resistance material layer, the data retention characteristics of the PRAM device may be determined by the time to keep the variable resistance material layer in the amorphous state or the crystalline state.
The variable resistance material layer of a PRAM device may include a calcogenide compound such as germanium (Ge), antimony (Sb), or tellurium (Te). However, the crystallization rate and the data retention characteristics of the calcogenide compound may be inversely proportional to each other.